The present invention relates to methods and apparatus for dry etching GaAs, and the like, and, more particularly, to anisotropic chemically enhanced etching apparatus characterized by its ability to produce low damage in the surface being etched and comprising, an evacuated main chamber in which an etching process takes place; a stage for receiving and holding a substrate to be etched within the main chamber; duct means for directing a flow into the main chamber and against a substrate to be etched mounted on the stage; cracking means connected to a supply of molecular chlorine on an input side and connected to the duct means on an output side for exciting and disassociating the molecular chlorine to be discharged through the duct means; first control means operably connected to the cracking means for controlling the level of .mu.W power into the cracking means and the flow of molecular chlorine into the cracking means whereby the chlorine radicals being emitted from the duct means are controlled; ion gun means for producing a beam of ions and for directing the beam of ions against a substrate to be etched mounted on the stage; and, second control means operably connected to the ion gun means for controlling the beam of ions.
Dry etching has gained popularity in recent years for the fabrication of Sielectronics and optoelectronic devices in the III-V compounds. These dry etching processes as applied to these particular uses have been developed primarily over the past decade. Despite many approaches having been taken by those skilled in the art as the art has developed to its present state, damage to the surface of the material being etched still remains as a major problem.
Sputter-etching or ion-milling has been used over an even longer period of time, and it still enjoys some use today, although many in the art have turned to some sort of reactive dry etching process in an attempt to reduce damage, increase anisotropy, reduce mask erosion, and achieve deeper etched features. As depicted in simplified form in FIG. 1, the intent is a common one. A mask 10 defining a pattern to be achieved is placed over a substrate 12 to be etched with the pattern. An etching beam 14 of some type is then directed against the mask 10 and exposed areas of the substrate 12. The beam 14 is stopped by the mask 10 thereby causing the exposed areas of the substrate 12 to be removed by the etching action of the beam 14. As will be discussed in greater detail shortly with respect to the examples thereof depicted in FIG. 2, the "beam" 14 may comprise a stream which physically attacks the substrate 12, a stream which chemically attacks the substrate 12, or a combination of the two. For example, as shown in FIG. 2(a), in Ion Beam Etching (IBE) a stream of Ar.sup.+ ions from an ion gun 16 are directed against the substrate and the incident ions striking the substrate physically "mill" the material away on an atomic level; that is, the ions dislodge atoms of the substrate material in much the same way as a sandblasting gun would dislodge material on a larger scale.
The first reactive etching of III-Vs was done at AT&T Bell Laboratories about ten years ago. Integrated optics were, and remain, a key application of this kind of etching, although it has since been applied to numerous electronic circuits as well. In fact, integrated optics is an increasing field of use which is constantly pushing the art to its maximum as uses thereof are recognized. For example, in the growing field of optical computing, requirements of the total technique require the ability to form diode lasers as part of the overall computer structure and its interconnections on a single chip. As can be appreciated, such integrated optics must be highly accurate (to "aim" the laser beam produced from stage to stage on the circuit) and must have highly polished and minimally damaged surfaces associated therewith to minimize optical losses which could make the circuitry inoperative.
The first reactive etching efforts relative to optoelectronics made use of Cl.sub.2 -O.sub.2 -Ar gas mixtures for the construction of laser facets and narrow etched grooves to create coupled-cavity laser structures. Shortly after the first lasers with such Reactive Ion Etched (RIE) facets were reported, the first Reactive Ion Beam Etching (RIBE), as depicted in FIG. 2(d), was demonstrated. In both RIE and RIBE, chlorine ions are accelerated in a straight line to the substrate surface. The substrate is also assumed to be covered by an adsorbed layer of chlorine. The combination of incident ions and the adsorbed molecules result in a chemical reaction between the III-V and chlorine atoms. The reaction products are also sputtered away by the incident ions. Thus, etching occurs only along the path of the ions. With RIBE, the substrate is in a separate chamber from where the ions are generated and accelerated. Thus, angled walls (i.e. "facets") are easily created. Other prior art work has shown that this angled etching can also be accomplished in an RIE system if the substrate is enclosed in a Faraday cage.
The above-described etching work formed the basis of what is probably the most successful and widely publicized use of dry etching for optoelectronic device to date--the RIBE dry etching work of Asakawa et al. at the Optoelectronics Joint Research Laboratory. This apparatus included an Electron-Cyclotron-Resonance (ECR) microwave plasma source and surface-analysis equipment in an ultra-high vacuum system. The ECR source is an efficient and reliable method of generating plasmas in reactive gases. It also creates a higher percentage of reactive species which enhance the etching. The analysis equipment allowed the evaluation of the amount of surface damage as a function of various etching parameters for the first time. This lead to a recent report of the finding that the higher the sputter yield (number of etched atoms per incident atom), the less the damage for a fixed ion energy.
Some time before the above-described work of Asakawa et al. was begun, Geis et al. at MIT Lincoln Labs demonstrated an Ion Beam Assisted Etching (IBAE) process, as depicted in FIG. 2(b), in which chlorine was separately admitted to the chamber of an ion mill through a pipe 18, rather than directly into the ion gun 16 as in the RIBE approach. In this case, the Cl.sub.2 molecules are adsorbed to the substrate surface, and the etching is promoted by the incident Ar.sup.+ ions. It is thought that these ions perform two functions. First, that they stimulate the chemical reaction. Second, that they bombard away the reaction products. A little later, the same group also demonstrated a so-called "hot-jet" etching technique (see FIG. 2(c)) in which the chlorine was cracked in a small high temperature oven 20 as it entered the etching chamber. In this process there is a key difference. The chemical reaction takes place without the need for the high-energy ions since the chlorine is already cracked into its more reactive atomic form. Although spontaneous etching occurred at room temperature, by heating the substrate, the group found that the etching was enhanced since the reaction products were made more volatile. The Ar.sup.+ ion beam was necessary to clean the substrate initially, and it also could be used to etch away the reaction products. In a similar approach, Asakawa has also reported the use of microwave ECR 22 without an accelerating field in Radical Beam Etching (RBE), as shown in FIG. 2(e). In this case, chlorine gas molecules are cracked and excited to higher electronic states as they enter the vacuum system as in hot-jet etching as described above. The low-energy, highly-reactive species then react spontaneously with the III-V substrate; and, if the temperature is sufficiently high, the reaction products are volatilized and pumped away. This so-called "radical beam" etching provides crystallographic etching very much like wet etching. From a comparison of these prior art approaches, it appears that the ECR microwave source may be a more practical way to create the cracking and excitation of chlorine molecules in an etching process utilizing them.
As described above, a number of techniques have been proposed to create reactive etching species. The hot-jet etching technique of FIG. 2(c) has been demonstrated to etch GaAs spontaneously without the need for ion bombardment. In that case, the Cl.sub.2 gas is thermally disassociated into atomic radicals in a small tube furnace before it reaches the substrate. The Cl.sub.2 appears to be "cracked" most efficiently into atomic radicals (Cl*) by employing an Electron Cyclotron Resonance (ECR) plasma such as in the Radical Beam Etching (RBE) process of FIG. 2(e). Also, the ECR cracker is more controllable, more clean, and more reliable. If the substrate is heated, nonvolatile etch products are more effectively desorbed to provide surface cleaning and crystallographic etching. If the etch products are removed by ion bombardment instead as in the Reactive Ion Beam Etching (RIBE) process of FIG. 2(d), then the wall angle is determined by the ion beam direction, making anisotropic etching possible. In this technique, the ion source (i.e. ion gun 16) supplies the ions (Cl.sup.+) as well as the reactive flux (Cl*) so the physical and chemical components are coupled together; thus, control over the degree of chemical enhancement is difficult. The Ion Beam Assisted Etching (IBAE) approach of FIG. 2(b) offers some independent control of the chemical and physical components by delivering the etch gas to the sample separately from the ion beam; however, the degree of chemical etching is somewhat limited because the etch gas is not plasma excited and is relatively unreactive. Even more recently, Magnetron-plasma Ion Beam Etching (MIBE), not shown, was developed to allow for the decoupling of the chemical etch component from the ionic component. The neutral reactive species were obtained by Magnetron reactive Ion Etching (MIE) and the ion species were generated by an ion beam souce; however, as in RIE, there is still a physical component in the absence of the ion beam due to the presence of the dc self-bias (.about.50 eV).
Wherefore, it is the object of the present invention to provide a method and associated apparatus for dry etching based on a modified IBAE approach in which the etch gas (Cl.sub.2) is cracked into radicals in a microwave discharge and delivered in high concentrations to the etching stage through an adjacent nozzle independently from the Ar.sup.+ ion beam.
It is another object of the present invention to provide a method and associated apparatus for dry etching based on a modified IBAE approach which provides separate control of the radical flux and the ion flux.
It is still another object of the present invention to provide a method and associated apparatus for dry etching based on a modified IBAE approach which enables the degree of anisotropy and chemical enhancement to be varied over the widest possible range.
It is yet another object of the present invention to provide microwave apparatus in which the etch gas (Cl.sub.2) is cracked into radicals in a microwave discharge in an optimum and cost effective manner and which minimizes recombination of the radicals into atoms prior to the etching site.
It is still a further object of the present invention to provide a method and associated apparatus for dry etching in which the surface being etched is subjected to a minimum degree of damage by the etching process.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in combination with the drawing figures which accompany it.